The present invention describes block-resistant, radiation-curable coating systems based on high molecular mass, aqueous polyurethane dispersions, a process for preparing them, the use of the coating systems as paints and/or adhesives, and also articles and substrates provided with these paints and/or adhesives.
Radiation-curable, aqueous coating systems based on polyurethane polymers find application in the coating of substrates including wood, plastics and leather, and are distinguished by a multiplicity of positive properties, such as good chemical resistance and mechanical stability. A particular advantage is the ultra-rapid curing of the polyurethane top coat through crosslinking of the ethylenic double bonds present in the polymer, with the aid of high-energy radiation.
For many applications, as in the case of wood/furniture coating or the coating of plastics, for example, an important part is played by substantial initial physical drying after the water has evaporated but before radiation curing takes place. Substrates, which prior to radiation curing, have a film coating which is still tacky cannot be stacked, and even the slightest mechanical stresses, of the kind occurring in a paintshop in the course of transport, can leave marks and damage on the coating. Correspondingly, for the transport and storage of substrates which have been coated but not yet radiation-cured, the coater is required to operate at high levels of cost and complexity, possibly, to accept products which are outside specification.
Prior to radiation curing, systems known to date which exhibit good physical drying are nevertheless still very sensitive. The reason for this is that the high atmospheric humidity of the kind usual in paintshops which process water-based binders causes the films which have not been radiation-cured to start to swell and become soft. This leads in turn to the advantages of the block-resistant coatings, such as ease of handling and storage in stacks, being lost again.
It would be desirable to obtain coatings which are block-resistant after physical drying and whose water resistance is sufficient that they remain block-resistant even under high atmospheric humidity, or offer a certain degree of resistance in the event of incomplete radiation curing, for example, on articles of complex shape, such as chairs. The use of such coating systems will increase the efficiency of the painting operation and hence lower painting costs.
The requirements imposed on modern-day coating systems are highly diverse. Hence it is not only of advantage if radiation-curable coating systems are block-resistant after physical drying; instead, the film obtained after radiation curing ought to be distinguished by high chemical stabilities and good mechanical robustness.
EP-A 753 531 describes urethane acrylate dispersions based on hydroxyl-containing polyester acrylates and polyepoxy acrylates. Although the films described therein dry physically, their resistance to water prior to radiation curing is not good, and they also have weaknesses with respect to solvents after radiation curing. In the application, alcohols or amines having a functionality of three or more are not used to construct a high molecular mass polyurethane acrylate or to improve the block resistance in a deliberate way.
EP-A 942022 describes urethane acrylate dispersions based on hydroxyl-containing polyester, polyether or polyurethane acrylates in combination with polyepoxy acrylates. The coating systems described lead to physically drying clearcoat materials, but the water resistance prior to radiation curing is inadequate. Moreover, owing to aromatic constituents, the use of polyepoxy acrylates leads to poor weathering stability. The use of polyepoxy acrylates likewise leads to a brittle film after radiation curing, and hence to poor adhesion to plastics substrates.
Aqueous, radiation-curable polyurethane dispersions based on hydroxyl-containing polyester acrylates and polyether acrylates are found in EP-A 872 502. The coating systems lead to films which dry physically but exhibit poor block resistance. The solvent resistances are likewise inadequate. The use of alcohols or amines with a functionality of three or more for deliberately improving the block resistance is not described.
It was an object of the present invention to provide coating systems based on aqueous, radiation-curing polyurethane dispersions that after physical drying and before radiation curing provide coatings having good block resistance. These coatings, even without radiation curing, possess sufficient water resistance, so the block resistance is retained even in the event of heightened atmospheric humidity. Moreover, after radiation curing, the film coatings are mechanically robust and highly resistant to chemicals.
This object has been achieved through the use of amines and alcohols with a functionality of three or more for preparing the polyurethanes present in the dispersions.